Veins of the Brain and Spinal Cord.
—The veins of the brain form a large number of small vessels which pour their blood into larger veins lying in folds of the dura mater; these are known as the venous sinuses of the dura mater. These sinuses communicate with the venous plexus about the orbital fissure, coming from the posterior facial vein; with the internal jugular through the jugular foramen, and with the vertebral veins, in the vertebral canal. The chief sinuses of the dura mater on the dorsal side of the brain are as follows:
1. Sinus sagittalis superior.—This lies in the dorsal part of the falx cerebri, between the two hemispheres of the cerebrum. It receives veins from the dorsal and middle parts of the cerebrum, and passes caudad to the tentorium. Here it enters the sinus transversus. It receives the vena cerebri magna, a large vein coming from the interior of the brain and passing directly dorsad at the caudal end of the corpus callosum to enter the sinus sagittalis.
2. Sinus transversus.—This lies in a canal in the dorsal border of the tentorium. It receives numerous small veins from the cerebellum, roof of the skull, etc. One or two centimeters on each side of the middle line the sinus transversus passes out of the canal onto the caudal surface of the tentorium, thence proceeds caudoventrad obliquely over the surface of the cerebellum, unites with veins from the ventral side of the brain, leaves the skull by the jugular foramen, forming thus the inferior cerebral vein, and joins the internal jugular vein.
On the ventral side of the brain are the following sinuses of the dura mater:
3. Sinus cavernosus.—A short broad venous sinus, one on each side of the hypophysis, on the body of the sphenoid. It receives veins from the side and ventral surface of the brain. The two sinuses are connected by communicating branches craniad and caudad of the hypophysis. From them branches pass out through the orbital fissure to join the plexus formed by the branches of the posterior facial vein.
4. Sinus petrosus inferior.—This arises from the sinus cavernosus and passes caudolaterad in the groove between the edge of the petrous bone and the basilar portion of the occipital. Reaching the jugular foramen it divides; part joins the termination of the sinus transversus to form the inferior cerebral vein, which passes through the jugular foramen to join the internal jugular vein. The other portion of the sinus petrosus inferior passes through the condyloid canal of the occipital bone, communicates by a strong transverse branch across the surface of the basioccipital with the vein of the opposite side, and enters the spinal canal through the foramen magnum. Here it joins the sinus of the vertebral column.
5. Sinus columnæ vertebralis.—On the ventral surface of the vertebral canal, beneath the periosteum, are two wide venous sinuses, one on each side of the middle line. These sinuses extend the entire length of the spinal cord. At the atlantal foramen each sends a strong branch to the internal jugular vein. Farther caudad they send branches to the vertebral, intercostal, and lumbar veins, and communicate with each other by numerous transverse branches. They receive many small veins from the spinal cord.
3. Vena cava inferior ([Fig. 129], c; [Fig. 126], a).
The inferior vena cava ([Fig. 126], a) is formed at about the level of the last lumbar vertebra by the union of the two common iliac veins ([Fig. 126], w). It passes craniad near the dorsal median line, lying at first dorsad of the aorta (b), then to the right, then ventrad. It enters the substance of the liver in the dorsal part of the caudate lobe of the latter, passes through the liver, and then through the diaphragm near the ventrolateral edge of the central tendon. It passes then craniad in the thoracic cavity ([Fig. 129], c) ventrad of the caudal lobe of the right lung, and enters the right auricle.